How to stop fluid simulation to stick with moving obstacle after collision
Thanks to Nick Bosse 18,
So the solution is to check 'remove air bubbles' in Physics -> Fluid -> Fluid Boundary.
*Note: After I did this, the effect of the frog leaping into the water doesn't look as 'strong' as before but I have bubbles emitting from it. So I keyframed the size of the bubbles from big ones when it enters the water, to smaller ones when it's swimming.